Often spelling errors or I realize I’ve opened myself up to a nitpick and I want to nip the nitpick.
Stupid typos - I original write “had an orange” and changed it to “ate an orange” but forgot to delete “had” so it now reads, “had ate an orange”.
I usually do a spell check before hitting submit, but as most are well aware, spell check doesn’t catch stupidity.
Usually it’s to retract the highly inflammatory and counter-factual statement I made, after onlookers have had a chance to be contaminated by its evil and polluting logic, but before they’ve managed to preserve it by quoting in a reply.
That’s what we were all afraid of, in all those years when editing simply wasn’t allowed, isn’t it?
Actually, I edit to remove silly mistakes I only spotted after posting for example spelling mistaeks or repetition, for example.
Should have been originally…
That is how we used to correct errors, by cluttering up the threads with apologies for typos.
I think you mean: “should’na”.
My edits are usually for one of the following reasons:
My two attempts at previewing I read what I meant to type, not what was actually on the screen, and so articles are often missing, and sometimes redundant words abound.
Punctuation. I think about 20% of my edits are because I forgot to include a closing comma, or parentheses for a parenthetical statement.
Links. I usually add links just before I submit my post. There are times that I forget, when submitting my post to actually include the link. Very embarassing if I’m starting a Pit thread about some RO story.
Sometimes I edit to add a related thought that hit me after the original post went out, or to respond to a new post.
But usually, it comes down to: I’m editing because I try to pretend on the interwebs that I’m not a fuckwit. 
In order:
Spelling
Grammar
Punctuation
Someone else has got there first (on preview) and said it better than I could
Someone else has said something I want to reply to (on preview)
My links don’t work
Of course, usually by the time I realise I need to edit, my window has elapsed and I look like an idiot for eternity.
Oh, and to fix broken tags.
I usually use preview for anything longer than a sentence or two, but will edit shorter posts for the usual reasons: correcting errors of spelling or grammar, correcting effed up coding, or adding an afterthought.
Would’n you no!
This morning I was post #51 on a thread so I topped page 2. After posting I noticed that the Google ads were all related to confusion with sexual orientation. Huh?!
Fixing typos is number one. I have to say, maybe it’s Word’s AutoCorrect, maybe it’s Google’s ‘Did you mean…’, but my typing has really gone downhill over the past few years. It’s like my lazy fingers have figured out they can just type all the required letters in any old order they please.
Spelling, changing what I would say into what I should write and pronoun problems.
Because I post almost exactly what I would be saying if we were speaking face to face, I often have to go back and take out repeats (things like “he was in back of the house so we went around the back”). I’ll also go back and put proper nouns where I had pronouns because I’m sure no one can follow what I’m thinking in my head.
Also, I’m the world’s worst speller.
Hey! I take offense to you calling me a dingbat! If you aren’t calling me a dingbat, then I’m offended that you didn’t include me with the dingbats!
fixing typo’s
fixing grammar, but that only gets really wrong when I excessively edit sentence structure.
adding clarity
fixing coding
I edit often for spelling, grammar and afterthoughts. Sometimes I respond immediately to a post before reading to the end of the thread and I tack another reply on. Occasionally I need to fix coding.
Biggirl, I might be a worse speller than you. I am sure I was when I joined the SDMB.
Usually typos, usually of the “th echair” variety (getting the space between words and the first letter of the next word out of order.
Also, adding a thought I missed the first time, sometimes qualifying something.